The present invention relates to an evaluating circuit for a data stream transmitted in the bi-phase code.
In digital data systems, the type of code used is an important factor. For the conversion from analog to digital values, for example, different codes may be employed (e.g., BCD or Gray code). Also, the question as to whether the possible states (mostly two) should be characterized by different currents, voltages, frequencies or the like can be answered differently depending on the particular requirement. Successive data pulses give a data stream in which different states follow each other in time (during transmission) or in space (on a recording medium). The information may be contained either in the successive states or in the changes of state. The latter system is mostly employed if simple, AC-coupled amplifiers are used, or if the clock must be recovered in the transmitted data stream. To recover the data from a transmitted data stream, it is necessary to determine whether a change of state occurs in one direction (e.g., as a voltage rise) or in the other (e.g., as a voltage drop) at a given instant. Accordingly, if the same value is to be transmitted twice in succession, there must be two changes of state in the same direction. In the meantime, there must necessarily be a change of state in the opposite direction. This additional change of state must be detected and suppressed during evaluation. This requires a fixed time (or spatial) distance between the valid changes of state. Any additional changes of state which may be necessary but are invalid for the data contents will then lie approximately midway between two valid changes of state and, thus, can be detected and suppressed if the data clock is known.
The data stream representation just mentioned is called "bi-phase code"; the states occurring are referred to as "phases", and the changes of state as "phase changes".
To store data in connection with so-called home computers, use is frequently made of tape recorders as are commonly employed in entertainment electronics. Such recorders exhibit flutter. To recover the clock required to recover the data, it is common practice to insert, at regular, relatively short time intervals, synchronization intervals with which a free-running clock generator on the evaluation side is synchronized. To generate and store this additional synchronizing signal, additional circuitry and, above all, additional type material are required.
With a known circuit, the CD4037A of RCA, the data transmitted in the bi-phase code can be recovered if the clock is known. However, this circuit suffers from the drawback that the phase changes that are insignificant for the information content are not completely suppressed. The result in short-duration interference pulses at the output.